There are a number of concerns which must be addressed in the design and fabrication of circuitry for power amplification, such as the power amplifier of a wireless communication device. For such a device, the concerns include ensuring sufficient gain, providing efficiency with respect to converting direct current (DC) power to radio frequency (RF) output power, establishing breakdown voltage conditions that are sufficiently high to enable long term use of the device, and achieving reliable on-off performance of switching circuitry in switching-class power amplifiers. Currently, there is a desire to use low cost, standard digital complementary metal oxide semi-conductor (CMOS) circuitry for radio functions. This desire magnifies potential problems, because CMOS circuitry typically has very low breakdown voltages.
There are two modes of breakdown voltages which should be considered. The first type of breakdown is junction breakdown. Excess electrons or holes are generated by high electric fields, creating an unwanted flow of current across the device. Eventually, a point is reached where the current actually increases, even as the voltage begins to drop (due to discharge of the anode). This “negative resistance” action allows an increasing current to flow, until excessive heat is generated. Eventually, permanent damage will occur. The second type of breakdown is across an oxide. In MOS processes, the gate of a transistor is insulated by an oxide layer from its drain, source and bulk nodes. Whenever a forward voltage is placed on the gate, there is a potential for breakdown across the oxide, in which the gate can short to the source, drain or bulk regions of the MOSFET. Even if no breakdown occurs across the gate, a long-term threshold voltage shift can occur, which causes the characteristics of the MOSFET to shift, if the gate-source voltage is kept too high for a long period of time.
Power levels commonly used in wireless RF communication devices can result in relatively large voltage swings. For example, at a power level of 4 watts, in order to obtain +36 dBm of transmitted output power on 50 ohm transmission lines, a signal of 40 volts, peak-to-peak may be required. It is likely that conditions are worse for poorly matched loads that are not at the nominal 50 ohm load impedance. The large voltage swings are a problem for modern, high speed semiconductor devices, which typically operate at power supply voltages of only a few volts, with the situation being particularly problematic for sub-micron CMOS integrated circuits which must operate at very low power supply voltages. Part of the problem results from the need to efficiently convert DC power to RF output power. For a single-ended power amplifier circuit running in class A mode, the efficiency may be approximately 50 percent. The class A amplifier is very linear and relatively free of distortion, but is less efficient than a class B amplifier, wherein the efficiency may be 78 percent.
In both the class A and class B modes of operation, transistors of a power amplifier have a linear relation in terms of input-to-output power. This linear operation generally results in a somewhat lower efficiency. If the transmitted signal is constant envelope (or if a modulator is used to take advantage of polar modulation methods), non-linear switching mode amplifiers may be used. One example of such an amplifier is the class E amplifier, which operates as a switching amplifier. That is, the transistors of class E amplifiers operate as switches, turning “on” and “off” during operation. In the case of class E amplifiers, a matching network may be employed to ensure that the switch only operates when the voltage across the transistor is zero, so that there are minimum losses during the switching transitions. This mode of operation can allow efficiencies approaching 100 percent. A class D amplifier is another switching-class power amplifier that works by adjusting its duty cycle in proportion to the input waveform. Unfortunately, while the switching-class power amplifiers are highly efficient, they tend to have lower gain than class A or class B amplifiers. When the gain of the power amplifier is low, it requires more power from the input to turn “on” the output device. This input power reduces the efficiency of the RF system in which the power amplifier is a part. For this reason, the term “power added efficiency” (PAE) has been used as a more accurate reference to the efficiency, since the measurement takes into account input power needed to operate the switches. In general, power amplifiers with higher gain have higher PAE.
Another categorization of power amplifiers is one in which the amplifiers are identified as having either a single-ended configuration or a differential configuration. In the single-ended configuration, a single input signal, generally referenced to ground, is amplified. In comparison, differential amplifiers amplify the voltage difference between two input signals. One deficiency of the single-ended amplifier is the fact that the connection to ground for the source of the input transistor must pass through the inductance of a bond wire and package lead for the integrated circuit that includes the amplifier. On the other hand, in the differential configuration, a virtual ground exists at a common connection to the sources of the two input transistors. As a result, only DC current flows through the grounded bond wire from the sources. In practice, the current in the transistors is not exactly equal and opposite, but most of the beneficial effects are still achieved.
While the above-described configurations of power amplifiers operate well for their intended purposes, further advances are available.